Tire pressure
05-09-2019, 03:16 PM
#1
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Tire pressure
Headed to my first autocross with the above in about a week, its been years so one very rusty driver. What is a good tire pressure starting point for the RE71R ?
05-10-2019, 08:45 AM
#2
Its yust what you want.
If you want to use the tires afterward on the road, you must take care that not any part of tire reaches a temperature at wich it hardens permanently, and then next bendings by deflections crack the little cracks further , untill after mayby only 2 years that much damage builded up that tire blows and acident and injuries or death.....aaaaah.
I can help you determine lowest pressure for savety, but need tirespecifications like ,1 maxload or loadindex, 2. Standard load or XL to determine reference- pressure, 3 speedcode to determine speed for wich maxload is calculated( reference-speed).
From car , most tricky part, real weights on tires, and speed used, wich you wont go over for even a minute.
But even with that determined lowest safe pressure, racing is difficult to determine what gives no damage.
If you now change the tires after the race, you can accept the colateral damage, and go lower then safe in pressure for more gripp, wich is important for racing.
Certainly dont blead air when warm to give more gripp, if you want to use your tires on the road afterward.
If you want to use the tires afterward on the road, you must take care that not any part of tire reaches a temperature at wich it hardens permanently, and then next bendings by deflections crack the little cracks further , untill after mayby only 2 years that much damage builded up that tire blows and acident and injuries or death.....aaaaah.
I can help you determine lowest pressure for savety, but need tirespecifications like ,1 maxload or loadindex, 2. Standard load or XL to determine reference- pressure, 3 speedcode to determine speed for wich maxload is calculated( reference-speed).
From car , most tricky part, real weights on tires, and speed used, wich you wont go over for even a minute.
But even with that determined lowest safe pressure, racing is difficult to determine what gives no damage.
If you now change the tires after the race, you can accept the colateral damage, and go lower then safe in pressure for more gripp, wich is important for racing.
Certainly dont blead air when warm to give more gripp, if you want to use your tires on the road afterward.
05-10-2019, 10:33 AM
#3
Drifting
RE-71R are pretty stiff sidewall, so you don't need to add as much air as you might for a soft tire like MP4S. However, autocross puts a lot of stress on the sidewall. I've found Porsche's recommended cold pressures - once hot - are actually pretty close to my ideal autocross pressures (987.2S).. You might need to add maybe 5psi or so so you're at the right hot pressure for the first run, and go from there. A lot depends on your driving style, too.
I found I need to run much higher pressures for autocross than for track.
?? Not that big a deal to put a few lbs back in later. I've bled out as much as 10psi some events to keep good grip. If you're not bleeding air to stay at ideal pressures, you're losing a lot of time for nothing.
I found I need to run much higher pressures for autocross than for track.
?? Not that big a deal to put a few lbs back in later. I've bled out as much as 10psi some events to keep good grip. If you're not bleeding air to stay at ideal pressures, you're losing a lot of time for nothing.
Last edited by sjfehr; 05-10-2019 at 12:13 PM.
05-10-2019, 11:49 AM
#4
On my '84 I ran around 30/38 for best results with the RE-71R. I did run a different size than you (17s,) but should be at least a decent starting point. Depending on your setup / driving / surface the car may like a little more or less than that.
Have fun!
Have fun!
05-10-2019, 12:41 PM
#5
Burning Brakes
Get a portable pump or reservoir, an accurate gauge, listen to Burglar and then experiment.
if it’s a hot day you may need a water sprayer to cool the tires. RE71R easily overheat , especially with low pressure and 2 drivers.
if it’s a hot day you may need a water sprayer to cool the tires. RE71R easily overheat , especially with low pressure and 2 drivers.
05-10-2019, 12:57 PM
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Its yust what you want.
If you want to use the tires afterward on the road, you must take care that not any part of tire reaches a temperature at wich it hardens permanently, and then next bendings by deflections crack the little cracks further , untill after mayby only 2 years that much damage builded up that tire blows and acident and injuries or death.....aaaaah.
I can help you determine lowest pressure for savety, but need tirespecifications like ,1 maxload or loadindex, 2. Standard load or XL to determine reference- pressure, 3 speedcode to determine speed for wich maxload is calculated( reference-speed).
From car , most tricky part, real weights on tires, and speed used, wich you wont go over for even a minute.
But even with that determined lowest safe pressure, racing is difficult to determine what gives no damage.
If you now change the tires after the race, you can accept the colateral damage, and go lower then safe in pressure for more gripp, wich is important for racing.
Certainly dont blead air when warm to give more gripp, if you want to use your tires on the road afterward.
If you want to use the tires afterward on the road, you must take care that not any part of tire reaches a temperature at wich it hardens permanently, and then next bendings by deflections crack the little cracks further , untill after mayby only 2 years that much damage builded up that tire blows and acident and injuries or death.....aaaaah.
I can help you determine lowest pressure for savety, but need tirespecifications like ,1 maxload or loadindex, 2. Standard load or XL to determine reference- pressure, 3 speedcode to determine speed for wich maxload is calculated( reference-speed).
From car , most tricky part, real weights on tires, and speed used, wich you wont go over for even a minute.
But even with that determined lowest safe pressure, racing is difficult to determine what gives no damage.
If you now change the tires after the race, you can accept the colateral damage, and go lower then safe in pressure for more gripp, wich is important for racing.
Certainly dont blead air when warm to give more gripp, if you want to use your tires on the road afterward.
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05-10-2019, 01:10 PM
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What I usually do is have what I think is extra air in there (air up the night before) and "throw away" the first run getting use to the course and warming up the tires........then bleed down to what I think the car wants.
05-10-2019, 01:14 PM
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Thank you. Its usually cold here and the pavement rough/dirty......I was guessing about 30 front and 35 rear. Shocks set on full soft.
05-10-2019, 03:47 PM
#10
So a run of 45 seconds, and then a pauze for mayby half an hour.
Then tires rubber has hardly the chanche to get a critical temperature.
Then you can go verry low in pressure, for best gripp.
Will have to calculate it, needing the data I asked, but probably about 18 psi cold pressure.
Cold pressure is simply, when in- and out-side tire temp are the same.
Then the warm pressure will stay low too, because inside tire aire also wont get the change to get much higher. If you have TMPS you can watch the pressure,- rising to check if tempn tire realy stays low, so most likely rubber to.
After that no permanent damage expected, so pump up to street- pressure and drive home.
Then tires rubber has hardly the chanche to get a critical temperature.
Then you can go verry low in pressure, for best gripp.
Will have to calculate it, needing the data I asked, but probably about 18 psi cold pressure.
Cold pressure is simply, when in- and out-side tire temp are the same.
Then the warm pressure will stay low too, because inside tire aire also wont get the change to get much higher. If you have TMPS you can watch the pressure,- rising to check if tempn tire realy stays low, so most likely rubber to.
After that no permanent damage expected, so pump up to street- pressure and drive home.
05-13-2019, 09:12 AM
#11
There's not enough sidewall support to run 18psi at AX. We run at higher slip angles than what a track car would run at. Lowest I've heard of a successful car running was ~25 psi in a light car with lots of camber, I had best results at nearly 40 psi on high grip surfaces in my camber challenged fwd car. Every case is a little different, of course.
05-13-2019, 10:44 AM
#12
Burning Brakes
Experimentation example on new-build ES 944 on Rival-S tires:
Co-driver (lots of 924/944 experience) and I started at 26 psi all around. (This is a balanced car, not a 911!)
Then we went to 28, then 30, then 32psi. 28 was better, 30 better yet. At 32 we were both clear the car was losing grip and it was clear that 26 was not enough.
Dropped back to 30 for final, best runs. Now I had a pretty good baseline pressure. This is all for best grip, not worrying about response, not paying attention to the times. (They change too much during the event for other reasons.)
Further experimentation at other events and I eventually settled on 31F/29R on high-grip sites. 30F/28R for my low-grip local site.
These are all "just before the run" pressures. Adjust pressure before each run... that's the only thing you can really control. Surface temps after the run change too quickly to be of much use. Below the surface temps (like road racers use) are useless for autocross also with these modern tires. (Once you are fast and using the "grip on the other side of slip" you have to keep carcass cool to keep surface from overheating in the Summer, especially with RE71R. Rival-S is more tolerant.)
The Porsche standard pressures for standard tires and standard alignment on the 944 are 29F/36R. 36psi in the rear is impossible for me to understand. In my 944 with autocross alignment and modern tires 36psi in the rear would clearly produce significant oversteer at the limit. Maybe back then it was necessary to reduce grip at the rear to balance a car that was otherwise set up for understeer or to keep the sidewall from rolling over given how much the car rolled in the corners. Who knows? (My car doesn't roll much.)
Now I have RE71Rs on 2" greater diameter wheels and wider tires on the same width. I will start the experimentation over again this weekend.
All 3 of the cars I've set up with various tires all ended up with "start of run" pressures in the neighborhood of the placard cold pressures for stock tires. For the track with street tires they all had to be started much lower (and carfully warmed -up) or the pressure would climb too high during the session. (note: I do a lot of trail-braking & sliding on corner entry.)
Co-driver (lots of 924/944 experience) and I started at 26 psi all around. (This is a balanced car, not a 911!)
Then we went to 28, then 30, then 32psi. 28 was better, 30 better yet. At 32 we were both clear the car was losing grip and it was clear that 26 was not enough.
Dropped back to 30 for final, best runs. Now I had a pretty good baseline pressure. This is all for best grip, not worrying about response, not paying attention to the times. (They change too much during the event for other reasons.)
Further experimentation at other events and I eventually settled on 31F/29R on high-grip sites. 30F/28R for my low-grip local site.
These are all "just before the run" pressures. Adjust pressure before each run... that's the only thing you can really control. Surface temps after the run change too quickly to be of much use. Below the surface temps (like road racers use) are useless for autocross also with these modern tires. (Once you are fast and using the "grip on the other side of slip" you have to keep carcass cool to keep surface from overheating in the Summer, especially with RE71R. Rival-S is more tolerant.)
The Porsche standard pressures for standard tires and standard alignment on the 944 are 29F/36R. 36psi in the rear is impossible for me to understand. In my 944 with autocross alignment and modern tires 36psi in the rear would clearly produce significant oversteer at the limit. Maybe back then it was necessary to reduce grip at the rear to balance a car that was otherwise set up for understeer or to keep the sidewall from rolling over given how much the car rolled in the corners. Who knows? (My car doesn't roll much.)
Now I have RE71Rs on 2" greater diameter wheels and wider tires on the same width. I will start the experimentation over again this weekend.
All 3 of the cars I've set up with various tires all ended up with "start of run" pressures in the neighborhood of the placard cold pressures for stock tires. For the track with street tires they all had to be started much lower (and carfully warmed -up) or the pressure would climb too high during the session. (note: I do a lot of trail-braking & sliding on corner entry.)
Last edited by edfishjr; 05-13-2019 at 11:23 AM.
05-16-2019, 11:48 AM
#13
Rennlist Member
Id say 32 rear and 30 front as a good starting point. Then adjust to assure you are getting full contact on tread. As mentioned these tires have very stiff sidewalls so not as much pressure as some tires required. If youre rusty tire pressures are probably not going to be the biggest performance concern for you.
05-16-2019, 01:26 PM
#14
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My last event was on RE's......years ago in an Evo
05-23-2019, 10:49 PM
#15
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Old fashioned method: put some white shoe polish stripes on the shoulder of the tread, and onto the outer 1 inch of the sidewall. After running, see how much the tire has rolled onto the sidewall. A little is OK, but lots on running on the sidewall is not. Adjust pressures accordingly.
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